A FOUR-LEVEL CLASSIFICATION SYSTEM FOR IRON LIMITATION IN COASTAL UPWELLING SYSTEMS

Iron limitation is now well documented in the three large oceanic HNLC regimes. Our new work extends this concept to a fourth major regime: coastal upwelling regions. Like other HNLC areas, addition of iron to low-Fe waters of the California upwelling region allows blooms of large diatoms and drawdown of ambient nitrate. Because shelf- and river-supplied iron inputs are unevenly distributed in both space and time, though, phytoplankton iron limitation is far from uniform. Consequently, the mutually exclusive designations "Fe-limited" and "Fe-replete" are completely inadequate to describe the dynamic complexity of the system.
In an attempt to address this problem, we present a four-level classification scheme for the degree to which California waters are limited by iron. In Type 1 Fe-replete areas, abundant inputs supply enough Fe that plankton growth and nutrient biogeochemistry are unaffected by Fe availability. In Type 2 Fe-stressed waters iron does not affect nutrient drawdown, but does control community species composition. In moderately Fe-limited Type 3 waters, diatoms are Fe-limited but smaller phytoplankton and bacteria are not. In Type 3 areas Si:N molar utilization ratios are elevated (2-3), leading to eventual co-limitation of aged upwelled waters by both Fe and Si, with NO3- remaining unused. In severely Fe-limited Type 4 waters, diatom growth is reduced so much that both Si and N may persist unused in surface waters, although the limited drawdown that does occur is also at a high molar Si:N ratio. In Type 4 areas Fe controls productivity by not only diatoms, but also directly or indirectly controls the growth of nanoplankton, bacteria and zooplankton.
Similar classification systems may also be useful in other regimes. For instance, Southern Ocean waters south of the Polar Front resemble Type 4 areas, while north of the front Type 3 conditions often prevail. It is time to recognize that Fe limitation is not an off/on switch but a continuum, with distinct biological and biogeochemical markers being characteristic of varying degrees of Fe availability.